Prior art heat exchangers and evaporative processes as employed for refrigeration and the like have recognized drawbacks which thus far have defied correction. Evaporators for refrigeration systems, air conditioning and other uses commonly employ an interior liquid running in a conduit whose walls transfer heat to the running liquid from an exterior fluid which may be gas or liquid requiring cooling. The interior liquid within the conduit undergoes evaporation and continually is converted into a gas. Until this conversion is complete, the interior running fluid is a gas and liquid mixture. The percentage of gas in the mixture increases until the interior fluid is all gas and no liquid and the evaporative process is completed.
In this gradual evaporative process, a gas bubble film tends to develop on the interior surface of the conduit for the running liquid and this film greatly hinders the transfer of heat through the wall of the conduit or tube to the liquid internally of the gas bubble film. In order to minimize this hinderance to efficient heat transfer, the interior running mixture must be propelled with a turbulent velocity to break up the gas bubble film in order to increase heat transfer efficiency. This, in turn, requires a greater consumption of energy.
Additionally, as the percentage of gas in the interior running fluid increases, the heat transfer hinderance factor correspondingly increases. For example, when the mixture becomes 60% gas and 40% liquid, the heat transfer rate in that part of the conduit drops to 40%, and in the area where the mixture is 90% gas and 10% liquid, the heat transfer rate drops to only 10%. Since a constant size tube or conduit is ordinarily employed in an evaporator, the average heat transfer rate all along the conduit is only about 50% of the true capacity of the heat exchanger or evaporator.
To increase the velocity and turbulence of the interior running fluid mixture not only consumes energy but increases internal friction which heats up the inside liquid. This obviously further decreases the ability of the system to transfer heat from the exterior fluid to the interior fluid. To cope with these two disadvantages, the heat transfer area (tube size) must be increased to increase the volume of internal liquid. It is also necessary to increase the energy of devices necessary for the removal of the interior liquid. In practice, a virtual dilemna is created. Because the exterior fluid such as air also has zones of unequal temperatures, the heat exchanger must simultaneously cope with unequal heat loads in different areas. This makes it impossible to choose a single efficient internal running fluid gas-liquid ratio. It follows from this that if a heavily heat loaded area of the exchanger would be cooled by a weakened liquid mixture, say 80% gas and 20% liquid, then, according to the above-explained process, the weakened liquid mixture and the lowest heat transfer capacity area will be asked to satisfy the heaviest heat transfer requirement which will be an impossibility. This phenomena compels the use of oversized heat exchanger components (a waste of material) and the maintenance of increased internal and external turbulent fluid flow (a waste of energy).
In addition to all of this, there is another inherent disadvantage in conventional heat exchangers concerning the interior working pressure determining temperature of evaporation of the liquid which is critical to system design. If the interior heat load rises, the inside liquid evaporating temperature also rises. As a consequence, the temperature differential between the interior and exterior fluids is diminished and this also requires additional enlargement of the heat transfer wall sides to meet requirements. The resulting over-dimensioning of the heat exchanger structure is wasteful of metal and labor.